Formerly, polarizing plates used in liquid crystal display devices generally had a structure in which a polarizer obtained by adsorbing elements to a polyvinyl alcohol film was sandwiched between two triacetyl cellulose films. However, since liquid crystal display devices have come to be used for TVs, varieties of cellulose ester films provided with retardation have appeared, for example, a cellulose acetate propionate film provided with an optical compensation function, and a film obtained by adding a retardation enhancing agent in a triacetyl cellulose.
The major reason why the provision of retardation to a cellulose ester was examined was that the cellulose ester had high affinity to the polarizer composed of a polyvinyl alcohol, resulting in a high productivity of a polarizing plate.
Such a cellulose ester film provided with retardation had a high degree of substitution with an acyl group in order to maintain moisture resistance of the polarizer.
However, in accordance with the improvement in the property of liquid crystal display devices,  it has become a situation in which it is difficult to fully obtain the necessary amount of retardation with conventional films, for example, a film obtained by incorporating a retardation increasing agent in a conventional cellulose acetate propionate film or a triacetyl cellulose.
Diacetyl cellulose which is a cellulose acetate obtained by decreasing the substitution degree of triacetyl cellulose (also referred to as cellulose diacetate) is easier to provide retardation when compared with a triacetyl cellulose. Accordingly, diacetyl cellulose has been known to be superior to triacetyl cellulose with respect to optical compensation, however, it has not been practically used because the problem of moisture resistance has not been overcome.
In view of such problems, in Patent document 1, proposed has been a technique to improve moisture resistance while exhibiting retardation by mixing a diacetyl cellulose with a compound having a negative birefringence and, for example, a simple sugar or an oligosaccharide.
However, according to this technique, the moisture resistance was improved, but not fully sufficient. Further, since the compatibility of the diacetyl cellulose with a simple sugar or an oligosaccharide was not enough, the internal haze or internal scattering of the film was enhanced, whereby the contrast of a liquid crystal display device when used was deteriorated.